I'm not a fan of the whole "shoulder disappearing/re-appearing" thing. I've seen people using it as a trick to get out of pressure. What they do is that say against a wrist grab, they take their shoulders out of their sockets --disconnect them from the upper center -- and create a void that hopefully would produce some momentum and movement for them to escape and re-direct the pressure. Doesn't matter how relaxed you think you are, it's not whole-body movement as the shoulder is acting independently and it won't affect an attacker who's centered; in fact by using the shoulder like that it gives the attacker a "handle" and easy exploit into nage's center. With weapons it's more severe as they could just drive through you if you provide such leeway into your center. Whenever somebody does this to me say during kokyu-dosa, I'm thinking OK, so you just freed your shoulders… good I guess?… but you're not doing anything
to me, in fact you just gave me an advantage, a "handle" into you. I think some students go to seminars and see teachers like Endo and explore movement based on copied external mannerisms but don't really know what else is involved.
The approach I've been thought is, keep shoulders in